Surface-emitting lasers that emit light in a direction perpendicular to a substrate are the subject of intense research and development. Surface-emitting lasers have a lower threshold current for oscillation than edge-emitting lasers, and are capable of emitting a high-quality circular beam profile. Because a laser output can be obtained from the surface-emitting laser in a direction perpendicular to a substrate of the laser, a plurality of the surface-emitting lasers can be readily integrated two-dimensionally at high density. Potential applications of the surface-emitting laser include a light source for parallel optical interconnection and high-speed/high-resolution electrophotography systems.
Typically, the surface-emitting laser has a confinement structure for improving current inflow efficiency. The confinement structure may be formed by selective oxidation of Al (aluminum), as discussed in Non-Patent Documents 1 and 2. Such a confinement structure may be hereafter referred to as an “oxidized confinement structure”.
Non-Patent Document 3 discusses a printer in which a VCSEL (Vertical Cavity Surface Emitting) array of a 780 nm band is used. Patent Document 1 discusses a surface-emitting laser in which a predetermined amount of a difference (“detuning”) is obtained between an oscillation wavelength which is determined by the length of a resonator and a peak gain wavelength which is determined by the composition of an active layer at a predetermined temperature. It is also discussed in Patent Document 1 that the oscillation wavelength corresponds to the peak gain wavelength in a temperature range above the predetermined temperature.
Patent Document 2 discusses a multi-spot image forming apparatus having a multi-spot light source. When a surface-emitting laser is applied to an image forming apparatus such as a printer, a small diameter of the beam spot focused on a photosensitive material is desirable. Further, for high-speed writing, a large laser output is desirable. Namely, it is desirable to obtain a high output in a single fundamental transverse mode (single-mode) operation.
In a printer system, for example, image quality is greatly affected by the behavior of the response waveform (“optical waveform”) of an optical output of a light source at the rise timing following the start of supply of a drive current, namely the changes in the optical output over time, such as the rise time. Such behavior includes the rise time. For example, image quality may decrease if the amount of light varies even slightly after the optical output has reached a certain amount in an initial period of the rise timing.
This is due to the fact that it is an outline portion of an image that is formed at the rise or fall timing of the optical waveform. Particularly, if the light amount varies at the rise timing or in a duration of time following the time that can be substantially considered the rise timing, the image outline is obscured, thereby rendering the image visually unclear.
When it takes 300 μs to scan one line on an A4-sized sheet along its length, which is about 300 mm, a width of about 1 mm is scanned in 1 μs. It is generally said that the human eye is most sensitive to a change in image density within a width of 1 mm to 2 mm. Thus, if image density changes in a width of about 1 mm, that density change is well recognizable by the human eye, thus giving the viewer an impression of an obscured outline.
FIG. 1 is a graph of an optical waveform plotted when a surface-emitting laser having an oxidized confinement structure was driven under the pulse conditions of pulse width of 500 μs and a duty of 50% (pulse period: 1 ms). As illustrated in FIG. 1, when viewed in a relatively long time scale, the optical output exhibits a peak once immediately after rising, and then decreases and stabilizes. Such a change in the optical output is due to the self-heating of the surface-emitting laser and generally referred to as “droop characteristics”.
The droop characteristics illustrate a comparison of the optical output immediately after pulse application (i.e., immediately after the rise of the optical pulse) and the optical output 500 μs after pulse application, and quantify the influence of heat on optical output.
A detailed analysis conducted by the present inventors has revealed that, when the droop characteristics of FIG. 1 are viewed in a much shorter time scale by enlarging an initial portion of the optical waveform near the rise timing, as illustrated in FIG. 2, a change in the optical output is present that is different from the droop characteristics.
Specifically, the waveform of FIG. 2 shows that the optical output does not fully rise even after 100 ns and may be considered to have fully risen only after about 200 ns, after which the waveform gradually rises until about 1 μs. This phenomenon (characteristics) has never been reported as far as the present inventors are aware. The time scale illustrated in FIG. 2 is about 1 μs immediately after the application of a pulse (immediately after the rise of the optical pulse) and is very short compared to the time scale of the droop characteristics. Such characteristics in the short time span are referred to herein as “negative droop characteristics”. The negative droop characteristics are basically not observed in conventional edge-emitting lasers.
A detailed analysis of the cause of the negative droop characteristics revealed that this phenomenon is due to a change in optical confinement in the lateral direction caused by the heating of the device, and a change in the light-emitting efficiency of the device.    Patent Document 1: JP2004-319643A    Patent Document 2: JP11-48520A    Non-Patent Document 1: K. D. Choquette, K. L. Lear, R. P. Schneider, Jr., K. M. Geib, “Cavity characteristics of selectively oxidized vertical-cavity lasers”, Applied Physics Letters, vol. 66, No. 25, pp. 3413-3415, 1995    Non-Patent Document 2: K. D. Choquette, R. P. Schneider, Jr., K. L. Lear, K. M. Geib, “Low threshold voltage vertical-cavity lasers fabricated by selective oxidation”, Electronics Letters, No. 24, Vol. 30, pp. 2043-2044, 1994    Non-Patent Document 3: H. Nakayama, T. Nakamura, M. Funada, Y. Ohashi, M. Kato, “780 nm VCSELs for Home Networks and Printers”, Electronic Components and Technology Conference Proceedings, 54th, Vol. 2, June, 2004, pp. 1371-1375